Difference between Network Address and Router Address

Question

I've been reading about IP addressing and came up with the following conclusions-

• A subnet mask divides an IP address into network address and host address.

• A network address always ends with a series of 0's like (mostly for all subnets) such as 123.0.0.0, 123.45.0.0 and 123.45.67.0.

• Routers act as intermediate stations to transfer data to the correct network.
  
  Now, I have the following questions-

• What actually is network address ? Does it identify any real world host or Router? i.e. are addresses like 123.0.0.0, 123.45.0.0 or 123.45.67.0 actually assigned to any routers or hosts.
• If they are assigned to routers then why is my local lan router identified as 192.168.0.1 which is not a network address (it doesn't have a trailing 0) and not as any other network address such as 192.168.0.0.
  
  EDIT
  
  Does it work this way,
  Suppose a request for 123.23.45.27 comes to a router, the router applies the subnet mask to it and suppose comes out with the network address as 123.23.0.0, so now it will check it's routing table for network address as 123.23.0.0 and refer to the corresponding router address entry that is capable of handle this request and that router further handles the requests.
  Are my following assumptions true-
  
  
• A router finds the Network address from the IP address and checks its routing table.
  
• The routing table contains entries in the form of Network Address vs Router address and then a matched router address is picked up for forwarding the request.
  
• All request's for IP address in the range 123.23.0.1 to 123.23.255.255 will be mapped to network address 123.23.0.0 and will be sent to the same router for further routing.
  
  Kindly help.

Answer 1

A subnet mask divides an IP address into network address and host address.

Correct. Apply the mask and you can see which part belongs to the network, and the reminder is the host address.

e.g. mask 255.255.255.0 and 1.2.3.4 would yield

IP:   00000001.00000010.00000011.0000100  (1.2.3.4. in binary)
Mask: 11111111.11111111.11111111.0000000  (mask in binary)
      -----------------------------------
Net:  00000001.00000010.00000011.0        (1.2.3.0)

A network address always ends with a series of 0's like (mostly for all subnets) such as 123.0.0.0, 123.45.0.0 and 123.45.67.0.

It often does, but part of that is habitual. I could use a netmask of 255.255.255.128 (Binary 11111111.11111111.11111111.1000000) to get smaller subnets. And 10.20.30.128 would be a network address, with 10.20.30.129 the first possible IP on it, 10.20.30.130 the second possible etc etc.

And for 10.20.30.0 to 10.20.30.127 the broadcast IP would be 10.20.30.127.

Routers act as intermediate stations to transfer data to the correct network.

Correct. They look at their routing table. If know have a specific route then they apply it. If not they forward it to their standard place. This is where the default gateway comes in.

Now, I have the following questions- What actually is network address ? Does it identify any real world host or Router? i.e. are addresses like 123.0.0.0, 123.45.0.0 or 123.45.67.0 actually assigned to any routers or hosts.

No. Network address are usually not assigned. They could be but then things would break down in most cases because the network address used to be used as the broadcast address before this got changed to the highest address in the network. Different implementations of IP stack would make using the network adress fun.

You might get away with it in very specific circumstances. E.g. on a specifc OS and with specific routers. But once you start to mix things, upgrade drivers or send things outside of your control (e.g. onto the Internet) you are going to have job security while debugging...

If they are assigned to routers then why is my local lan router identified as 192.168.0.1 which is not a network address

That is merely a normal IP. 192.168.0.0/16 is usually used as a /24, with usuable IPs from 192.168.0.1 though 192.168.255.254 (65536-2 IPs). With the first IP usually reserved and the last IP reserved (as broadcast address).

Most SoHo setups take the RFC1918 range of 192.168.0.0/16, split in into 256 smaller subnets (/24) and assign the router the first or the last usable IP.

This makes a lot of sense when you design a network. One of the first things you need to plan the a way off the local network, and giving that the first IP got to be custom. But that is pure custom. Giving the router and IP of 192.168.1.42 is equally valid.

Suppose a request for 123.23.45.27 comes to a router, the router applies the subnet mask to it and suppose comes out with the network address as 123.23.0.0, so now it will check it's routing table for network address as 123.23.0.0 and refer to the corresponding routing table entry that is capable of handle this request and that router further handles the requests.

Are my following assumptions true?

A router finds the network address from the IP address and checks its routing table.

True.

The routing table contains entries in the form of network address vs Router address and then a matched router address is picked up for forwarding the request.

Not sure about the versus. It contains network addresses. Briefly it does this:

• If the routers was the destination then the packet is handled up.
• If the router was not the destination then a router will check if it has a specific entry for a that host. Of it has it will forward it based on that.
• If the router was not the destination then a router will check if it has a specific entry for a network. If it has then it will forward it based on that.
• If the router was not the destination and it has neither of those then it will forward it to the default gateway as set on that router

All request's for IP address in the range 123.23.0.1 to 123.23.255.255 will be mapped to network address 123.23.0.0 and will be sent to the same router for further routing.

Only for a /16. There are 30 possible network masks (excluding 0.0.0.0 and host-to-host specific ones) and only 3 of those end on the common round .0's

Answer 2

An IP network consists of a range of IP addresses, such that it defines a:

Network ID (or Address) (eg 10.0.0.0/8): An address that can refer to the network as a whole. This is used by routers to route traffic between networks that are many hops apart. This address cannot be used by anything except routers routing tables (it is NEVER assigned to an interface; you cannot ping it, etc), for the purpose of finding a pathway between two networks. A network address must have an accompanying Subnetwork Mask to describe its scope (for instance there are hosts in the 4.0.0.0 network in New York City and in LA, so the netmask must be present to determine the correct path). Just remember, when thinking about Network Addresses, your perspective should be "I have lots of networks, how to I interconnect them all? I don't know or care whats in those networks. just how to get to and from them."

Broadcast Address (eg 10.255.255.255): an address that will send traffic to all hosts on the network automatically.

Host Addresses (eg 10.0.0.1 - 10.255.255.254): the unicast addresses that hosts use. A router on the 10. network has a unicast address (10.0.0.1?) and that router is said to "Service the 10.0.0.0/8 network". to be clear however, the router neither defines the network, nor Is the network; the network exists without it, but is unreachable from the outside world.

Gateway Address (eg 10.0.0.1): the address of a router on the same network, which connects to other networks. All inter-connected networks have a "Default Gateway" which is used if the computer does not specifically know of a router that can reach a remote network.

Note that I'm using 10.0.0.0 in my examples, because its easy to type, but these ideas are pervasive across networks of all sizes.

Answer 3

A subnet mask divides an IP address into network address and host address.

Correct.

A network address always ends with a series of 0's like (mostly for all subnets) such as 123.0.0.0, 123.45.0.0 and 123.45.67.0.

It ends with some number of zero bits. If there are eight of them, that will make the last octet a zero.

What actually is network address ? Does it identify any real world host or Router? i.e. are addresses like 123.0.0.0, 123.45.0.0 or 123.45.67.0 actually assigned to any routers or hosts.

They're assigned to the network. That's why they're called network addresses.

If they are assigned to routers then why is my local lan router identified as 192.168.0.1 which is not a network address (it doesn't have a trailing 0) and not as any other network address such as 192.168.0.0.

They're not assigned to routers, they're assigned to networks. That's why they're called network addresses.

Does it work this way, Suppose a request for 123.23.45.27 comes to a router, the router applies the subnet mask to it and suppose comes out with the network address as 123.23.0.0, so now it will check it's routing table for network address as 123.23.0.0 and refer to the corresponding router address entry that is capable of handle this request and that router further handles the requests.

That's not right. How would the router know the subnet mask associated with that IP address? The router knows the subnet masks of its routes, but not necessarily of arbitrary addresses.

The router finds all routes that match 123.23.45.27. If there are more than one of them, the router typically prefers the most specific match, that is, the one for the smallest network. If there are ties there, then various metrics (or load balancing rules) are used.

A router finds the Network address from the IP address and checks its routing table.

Nope. It can't do that. How would it know the subnet mask?

The routing table contains entries in the form of Network Address vs Router address and then a matched router address is picked up for forwarding the request.

Close. The routing table contains entries in the for of network address, subnet mask, and next hop which can be an address or an interface. For example, suppose this router is the last router before the host, the route can't hold the address of the next router -- there isn't one.

All request's for IP address in the range 123.23.0.1 to 123.23.255.255 will be mapped to network address 123.23.0.0 and will be sent to the same router for further routing

All IPs in that range will follow that route if that route is the most specific route for that range.